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1—
Introduction

The Coupled Sun-Earth System

The Earth environment as we know it exists because of the energy it receives from the Sun. Radiant energy from the Sun powers the atmospheric and oceanic circulations that profoundly influence the state of the biosphere. Without solar radiation, photosynthesis would cease. Solar radiation and high energy particles impinge continually on the envelope of gases and plasma that surrounds and protects the narrow habitable layer of the Earth's surface. Changes in the amount of solar energy input to the total Earth system are caused by three main mechanisms: i) geometric factors related to the Earth's inclination and orbit around the Sun (which alter the distribution of radiation incident on the Earth), ii) processes in the Earth system itself (which regulate the amount of energy received by the Earth), and iii) variations in the activity of the Sun (which modulate the energy emitted by the Sun).

Geometric relationships modulate solar inputs to the Earth. The seasonal progression of weather is controlled by the tilt of the Earth's axis of rotation relative to the direction normal to the Earth's orbital plane and by orbital eccentricity and precession. In addition, small periodic variations in the Earth's orbital parameters over time scales of tens of thousands of years (Milankovitch cycles) along with associated feedbacks



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Page 13 1— Introduction The Coupled Sun-Earth System The Earth environment as we know it exists because of the energy it receives from the Sun. Radiant energy from the Sun powers the atmospheric and oceanic circulations that profoundly influence the state of the biosphere. Without solar radiation, photosynthesis would cease. Solar radiation and high energy particles impinge continually on the envelope of gases and plasma that surrounds and protects the narrow habitable layer of the Earth's surface. Changes in the amount of solar energy input to the total Earth system are caused by three main mechanisms: i) geometric factors related to the Earth's inclination and orbit around the Sun (which alter the distribution of radiation incident on the Earth), ii) processes in the Earth system itself (which regulate the amount of energy received by the Earth), and iii) variations in the activity of the Sun (which modulate the energy emitted by the Sun). Geometric relationships modulate solar inputs to the Earth. The seasonal progression of weather is controlled by the tilt of the Earth's axis of rotation relative to the direction normal to the Earth's orbital plane and by orbital eccentricity and precession. In addition, small periodic variations in the Earth's orbital parameters over time scales of tens of thousands of years (Milankovitch cycles) along with associated feedbacks

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Page 14 and possible carbon dioxide changes are believed to cause significant variations in the Earth's climate. Processes within the Earth system regulate the solar energy inputs through numerous feedback mechanisms that influence the greenhouse warming of the Earth. Some of these feedbacks include variations in cloudiness and ice cover that determine the planetary albedo and hence affect the portion of the incoming solar radiation that is available to the Earth system. Variations in solar energy related to the activity of the Sun can also generate natural changes in the Earth system: assessing the extent of this latter effect is the topic of this report. There is no doubt that solar variability alters the energy input to the global Earth system, which is considered here in the broadest sense to extend from the biosphere, where weather and climate are experienced, to the Earth's near-space environment, some 1000 km above. Both the short-wavelength ultraviolet (UV) radiation and the solar wind and energetic particles from the Sun undergo large changes related to the presence of active regions in the solar atmosphere. These changes cause dramatic variability in the Earth's upper atmosphere, ionosphere, and magnetosphere. Only recently have spacecraft observations revealed that small variations (about 0.1 percent) also occur in the total electromagnetic energy radiated by the Sun. These radiative variations are also connected to the presence of active regions in the solar atmosphere (dark sunspots and bright faculae), and they occur on all time scales observed thus far, from minutes to the Sun's 11-year activity cycle. The spectrum of the radiant energy incident on the top of the Earth's atmosphere and the change in this radiation during the solar activity cycle are shown in Figure 1.1. Some of the Sun's radiant energy is reflected back into space by the Earth's surface, by clouds, and by aerosols; the remaining portion is absorbed by the Earth's surface and within the Earth's atmosphere. Figure 1.2 illustrates the altitude of unit optical depth. This is the mean altitude at which solar spectral energy is reduced by the Earth's atmosphere to roughly 1/e of its value at the top of the atmosphere. This curve is determined by the concentrations of radiatively absorbing gases in the Earth's atmosphere. Figures 1.1 and 1.2 indicate that the more variable, shorter wavelength solar energy is absorbed at higher altitudes in the atmosphere. Radiation at wavelengths shorter than

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Page 15 image Figure 1.1 (a) The Sun's spectral irradiance (solid line, typical of solar minimum conditions) is compared with the spectrum of a black body radiator at 5770 K (dashed line). The broad spectral bands identified along the top of this figure are the ultraviolet (UV), visible (VIS), and infrared (IR). Not shown, at wavelengths longer than the IR, is the microwave or radio portion of the solar spectrum. (b) Approximate amplitude of the Sun's spectral irradiance variation from the maximum to minimum of the 11-year activity cycle. The solar cycle variation in the spectrally integrated, or total, solar irradiance is indicated by the dot-dash line. From J. Lean, Reviews of Geophysics, 29, 506, 1991, copyright by the American Geophysical Union.

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Page 16 image Figure 1.2 Shown on the left is the altitude at which the solar irradiance is attenuated by 1/e (unit optical depth) in the Earth's atmosphere, for an overhead Sun. Also indicated are the primary atmospheric absorbing species of the radiation within different spectral bands and the wavelength regions that dominate ozone production and absorption. Adapted from Meier (1991). Shown on the right is the standardized temperature of the Earth's atmosphere from the surface to 250 km. Atmospheric regions, called spheres, are defined by boundaries based on inflections in the temperature profile (at approximately 15 km, 50 km, and 100 km) determined largely by solar radiative heating through gaseous absorption. Reprinted by permission of Kluwer Academic Publishers. about 160 nm is mostly absorbed above about 100 km (in the thermosphere), where solar variability generates temperature variations of hundreds of degrees. Solar radiation at wavelengths from about 150 to 310 nm is absorbed primarily in the middle atmosphere, which is conventionally defined as being that atmospheric region from about 15 to 100 km, between the troposphere and the thermosphere. Deposition of the Sun's energetic particle input to the global Earth system is more complicated. Here, interactions with the Earth's magnetic field are important since the charged particles in the solar wind are guided

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Page 17 along magnetic field lines. Thus, low latitudes are shielded from much but not all of the incoming charged particles, with most of the energetic particles being guided into the Earth's atmosphere in the polar regions. Numerous statistical studies have reported variations in atmospheric and hydrospheric parameters that are attributed to solar variability effects on many time scales. Generally speaking, it is much easier physically to relate variations in the Earth's upper atmosphere to known solar activity than is the case for the lower atmosphere and hydrosphere. This is because those energetic inputs from the Sun that show the largest amount of variability (associated with higher energy photons, solar wind, and energetic particles) are usually absorbed in the Earth's upper atmosphere. Solar forcing of the upper atmosphere is thus well recognized and has been verified by the agreement between atmospheric observations and theoretical assessments of the upper atmosphere's response to known solar energy inputs. Although energetically viable mechanisms for significant solar variability influences on the lower atmosphere and surface of the Earth have yet to be identified, some very interesting associations between solar variability and weather and climate have been suggested. Among these are the cited coincidence between the time of the Maunder Minimum (1645 to 1715) in sunspot activity and the coldest temperatures of the Little Ice Age Figure 1.3). Another example is the recent work by Labitzke and van Loon (1990, 1993) which suggests an association between solar activity, as measured by the 10.7 cm solar radio flux, and atmospheric temperature changes, with an important role being played by the phase of the quasibiennial oscillation (QBO) in the zonal wind in the tropical lower stratosphere. The main problem in quantitatively explaining statistical associations between solar variability parameters and sizable climate and weather effects is that the amount of energy in variable solar energy inputs is small compared both to the incoming solar energy itself and to lower atmosphere energetics. Table 1.1 compares the magnitudes of various solar and magnetospheric energy inputs to the Earth system. The total solar radiative energy input per unit area is about 1368 Watts per square meter (W/m2) in an averaged sense. Observed 11-year cycle variations in total solar irradiance (often referred to as the solar ''constant") are about 1.3 W/m2. This is two orders of magnitude larger than the averaged soft

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Page 18 image Figure 1.3 Relationship between winter severity in Paris and London (top curve) and long-term solar activity variations (bottom curve). The shaded portions of this curve denote the times of the Spörer and Maunder minima in sunspot activity. The dark circles indicate naked-eye sunspot observations. Details of the solar activity variation since 1700 are indicated in the bottom curve by the sunspot number data. The winter severity index has been shifted 40 years to the right to allow for cosmic ray-produced 14 C assimilation into tree rings. From J. Eddy, Science, 192, 1189, 1976, copyright by the American Association for the Advancement of Science.

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Page 19 TABLE 1.1 Comparative energy inputs from the Sun to the Earth system and the change in these energy inputs over the 11-year solar cycle. Also indicated are the approximate regions of the Earth system where the energy is deposited. Source Energy (W/m2) Solar Cycle Change (W/m2) Deposition Altitude Solar Radiation       total solar irradiance 1368 1.3 surface UV 200–300 nm 16 0.15 0–50 km UV 120–200 nm 0.1 0.015 50–120 km EUV 0–120 nm 0.003 0.005 100–500 km Particles       Solar protons 0.002   30–90 km Proton aurora 0.001 – 0.036   90–130 km Visual aurora 0.0006 – 0.6   90–130 km Galactic cosmic rays 0.000007   0–90 km Joule Heating of Thermosphere     E= 1 mV/m 0.000014   100–500 km E=100 mV/m 0.14   100–500 km Solar Wind 0.0003   above 500 km Downward Heat Conduction from Magnetosphere 0.00003   above 500 km X-ray and extreme ultraviolet (EUV) radiation inputs from the Sun and about one order of magnitude less than the solar ultraviolet (UV) radiation that enters into middle atmosphere ozone photochemistry. Solar UV radiation from 200 to 300 nm is believed to vary by a few percent, which implies that the energy associated with changes in this radiation is a factor of 10 or so less than that associated with total irradiance variations. Energetic particle sources posses still less energy.

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Page 20 The direct solar forcing of climate associated with the energy changes in Table 1.1 is currently thought to be smaller than is inferred from some of the observed statistical associations. This makes it difficult to develop viable quantitative models, since more complicated, indirect, amplifying or coupling mechanisms must be invoked. Nevertheless, the more energetic photons and particles that have the largest percentage variations are important candidates for forcing, since they can affect the concentrations of chemical constituents that can possibly redistribute larger amounts of energy. Energy from the Sun, whether as photons, energetic particles, or from solar wind-magnetosphere interactions, and whether deposited at low or high latitudes, is eventually distributed over the entire globe by the continuous motions of the Earth's atmosphere and oceans. Because of this, chemical, radiative, and dynamical perturbations generated by solar variability may be transported to different latitudes and altitudes; this absence of specific spatial and altitude boundaries within the global Earth system means that direct solar forcing of atmospheric regions remote from the biosphere may nevertheless affect it indirectly to some extent. Global Change Research The goal of the United States Global Change Research Program (USGCRP) is to establish the scientific basis for national and international policymaking relating to natural and human-induced changes in the global Earth system (Committee on Earth Sciences, 1989). To achieve this goal, the committee defined three specific objectives: i) establish an integrated, comprehensive, long term program of documenting the Earth system on a global scale, ii) conduct a program of focused studies to improve our understanding of the physical, geological, chemical, biological, and social processes that influence Earth system processes and trends on global and regional scales, and iii) develop integrated conceptual and predictive Earth system models.

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Page 21 Solar Influences on Global Change: A Major Scientific Research Element of the USGCRP The need to understand solar variability influences in the study of global change arises because solar-driven global change complicates the detection, understanding, and prediction of anthropogenic forcing. Research efforts toward achieving this understanding might be conceptualized thus: Monitoring : The solar energy inputs to the Earth system must be measured continuously. Solar phenomena (e.g., sunspots and faculae) that are thought to affect these energy inputs must also be measured. Changes in Earth system parameters must be monitored so that associations can be detected. Understanding : Once associations between different aspects of solar behavior are established, hypotheses are developed about their physical causes. The predictions of theories based on these hypotheses are then tested against observations in an effort to either prove or disprove the theories. Theories may have to be reformulated in light of new observations. The same intellectual process needs to be followed in formulating theories of the response of the Earth system to variable solar energy inputs. Agreement between theory and observation suggests that a good level of understanding has been achieved. Predicting : Two types of prediction are possible. One is statistical prediction. In this case, statistical associations are quantified by some sort of regression function which is then used to predict the future. A deeper level of understanding is required to make physical predictions. In this case, quantitative physical laws are formulated and their predictive capability is verified with retrospective data as well as by predictions into the future. The goal of research in solar influences on global change then is the development of the necessary data bases and understanding of the physical processes that lead to the ability to assess and predict the behavior of the Sun and its influence on the Earth system.

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Page 22 Objectives of the Report This report deliberately focuses first on the most obvious and immediate solar forcing of that part of the Earth's environment where life exists, where understanding solar influences on global change is most important to human welfare and which must thus have high priority. Chapters 2 and 3, therefore, concentrate on solar influences on temperature and composition of the lower layers of the Earth's atmosphere. Chapters 4 and 5 assess solar forcing of higher atmospheric layers and of the Earth's near-space environment and the possible coupling of this forcing to the biosphere. Chapters 4 and 5 do not attempt an exhaustive discussion of all solar-terrestrial connections; this is left, for the most part, to other studies. Chapter 6 discusses knowledge of solar variability itself. Chapter 7 covers strategies for research in solar influences on global change, and recommendations appear in Chapter 8. The Working Group on Solar Influences on Global Change met twice, in November 1990 and March 1991. Since then the topic has been the focus of three meetings: a Workshop on Solar-Terrestrial Impacts of Global Change, sponsored by the High Altitude Observatory in Boulder, CO, in May 1991; an international symposium on The Sun as a Variable Star: Solar and Stellar Irradiance Variations, International Astronomical Union, Colloquium No. 143, in Boulder in June 1993; and a NATO Advanced Research Workshop on The Solar Engine and its Influence on Terrestrial Atmosphere and Climate, in Paris in October 1993. Proceedings of these three meetings are in preparation. Significant effort has been made to include in this report the relevant results reported at these meetings and in the scientific literature, as of June 1994.